You may well have done so, but if not its worth contacting GlosFHS ?
http://www.gfhs.org.uk/Contactus.htm

They do have a Glos Monuments cd £10 which lists St John's as included.
http://www.gfhs.org.uk/glosmicd.htm

Depending where you're visiting from perhaps there Gloster Office is worht visiting first ?
According to the website open most days;

GLOUCESTERSHIRE FAMILY HISTORY CENTRE
Clarence Row, Alvin Street, Gloucester, GL1 3AH.
Telephone: 01452 524344
Email: help@gfhs.org.uk

The Centre is located in Gloucester City Centre within the grounds of Gloucestershire Archives.

Opening Times
Mondays to Fridays (except bank holidays) 10 am – 4 pm
1st and 3rd Saturday of each month 9.45 am – 12.45 pm

Good luck.

(Its a shame as last weekend I was a few yards from St John's visiting family)